High strength,ductile maraging steel



United States Patent 3,453,102 HIGH STRENGTH, DUCTILE MARAGING STEEL Glenn W. Tulfnell, Warwick, and Stephen Floreen, Suffern, N.Y., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Mar. 8, 1966, Ser. No. 532,597

Int. Cl. C22c 39/00 U.S. Cl. 75-123 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to maraging steels, particularly to maraging steels of improved yield strength and ductility.

In the relatively short period since their development and introduction, the maraging steels have already advanced to the point of exerting a significant influence in ferrous metallurgy. These steels, generally described in the U.S. patents to Bieber, No. 3,093,518, and to Decker, Goldman and Eash, No. 3,093,519, with the simplest of heat treatment and without recourse to cumbersome processing operations, such as plastic deformation by ausforming, afforded the highest combination of strength, ductility and toughness theretofore known. In addition, such steels (1) manifested high strength-to-weight ratios, (2) offered good castability and resistance to hot tearing characteristics, (3) eliminated need for quenching, (4) exhibited exceptional dimensional stability, (5) were machinable and weldable, (6) could be both hot and cold worked, etc. Against a background of such an imposing array of characteristics, it is readily understandable why a well publicized 18%-Ni maraging steel recently performed quite satisfactorily when tested (static) in the form of a 260 inch diameter rocket motor casing approximately 80 feet long. While this represents an achievement of no little moment, the incessant demand for materials capable of rendering improved performance leaves little doubt that steel of yet higher strength and greater ductility is the commercial necessity of the future, if not of the present. This is the case with thin walled pressure vessels capable of withstanding greater pressures, wire and cable of higher tensile strength, high energy absorption springs, heavy duty tools and dies, etc.

In reviewing the aforesaid U.S. Patent No. 3,093,519, of the more ductile and higher strength steels disclosed therein, to wit, those steels having yield strengths from about 270,000 pounds per square inch (p.s.i.) to 286,000 p.s.i., the average strength was 279,000 p.s.i. with a tensile ductility of about 10%. The highest reported yield strength for any steel set forth therein was 313,000 p.s.i., but the tensile elongation thereof, 4%, was on the low side. Even taking into consideration possible benefits conferred by way of improved melting and working techniques, such a combination of mechanical characteristics compares quite favorably with the 280,000 p.s.i. to 300,000 p.s.i. group of commercial maraging steels exhibiting tensile elongations of about 11% to 7%, the latter corresponding to the highest strength level. Now, the

problem to which the present invention is addressed involves providing a steel with the attributes of the maraging steels as above discussed but one which additionally substantially raises yield strength from the 270,000 p.s.i. to 300,000 p.s.i. plateau to the order of 350,000 p.s.i. (0.2% offset), e.g., 335,000 p.s.i. to 365,000 p.s.i., without comparable loss in ductility, preferably with enhanced ductility. The optimum would be to concurrently attain this desideratum without loss of notch toughness.

It has now been discovered that with a carefully balanced chemical composition of special constituents, including nickel, cobalt, molybdenum, titanium, aluminum and carbon, maraging steels of about 350,000 p.s.i. can be provided together with good ductility, including tensile elongation and reduction in area characteristics. In fact, yield strengths of such magnitude can be obtained with improved ductility and also good notch toughness. What perhaps is somewhat surprising, at least in retrospect and particularly in view of the tremendous amount of research work that has been carried on in respect of the maraging steels in virtually all quarters of endeavor, is that the steels of the subject invention fall within the broad disclosure of the above-mentioned U.S. Patent No. 3,093,519. Equally surprising is that although one aspect of the invention found necessary in reaching the stated objective features using a greater amount of hardening constituents than heretofore employed, this is achieved without sacrifice, comparatively speaking, of ductility. This is, as will be appreciated by those skilled in the art, quite inconsistent with expected or usual metallurgical behavior.

In any event, it is an object of the present invention to provide new and improved maraging steels capable of exhibiting yield strengths on the order of 350,000 p.s.i.

It is a further object of the invention to provide maraging steels of enhanced ductility (tensile elongation and reduction in area) together 'with a high level of yield strength circa 340,000 p.s.i. to 360,000 p.s.i.

It is also the purpose of the invention to provide notch tough, ductile, maraging steels having yield strengths of about 350,000 p.s.i.

Other objects and advantages will become apparent from the following description.

Generally speaking, maraging steels of the present invention consist essentially of, in percent by weight, about 15% to 20% nickel, about 8% to 16% cobalt, about 2.5% to 5% molybdenum, about 1.3% to 2.25% titanium, up to about 0.8% aluminum, the sum of the titanium plus aluminum being at least 1.6% and not more than 2.5 up to 0.05% carbon, up to 0.5% manganese, up to 0.5% silicon, and the balance essentially iron. In referring to iron as constituting the balance or essentially the balance of the steels, it is to be understood, as will be readily appreciated by those skilled in the art, that the presence of other elements is not excluded, such as those commonly present as incidental elements, deoxidation and cleansing elements, and impurities ordinarily associated therewith in small amounts which do not adversely affect the basic characteristics of the steels. Elements such as phosphorus, sulfur, hydrogen, oxygen and nitrogen should be kept at levels as low as is consistent with commercial steelmaking practice. Sulfur and phosphorus should not exceed 0.02% each and preferably should not exceed 0.01% each. Auxiliary constituents, which can be present, include the following: up to 1% vanadium, up to 1% columbium, up to 5% chromium, up to 1% tantalum, up to 2% copper, up to 0.2% beryllium, up to 0.01% boron and up to 0.1% zirconium. The total amount of these auxiliary elements should not exceed about 7%. Calcium, cerium and the like can be used in amounts up to 0.1% for purposes of deoxidation and the like.

In addition to the above compositional range, it is beneficial that the nickel, cobalt, molybdenum and titanium be specially correlated such that the following is satisfied:

20 (percent Ni15) +11 (percent Co-i? +68 (percent Mo-2.5)+66.5 (percent Ti1.3) 1s not greater than about 247.

By observing the above relationship, initiation of the transformation from austenite to martensite, to wit, the martensitic transformation temperature (Ms), will be above about 300 F. This is important in insuring substantial freedom from both retained austenite prior to aging and reverted austenite upon aging. Excessive austenite exerts a degrading influence and whether retained, or formed by reversion, it is advantageous that the amount thereof, if any, not exceed With further regard to the alloying constituents, should the nickel content appreciably exceed the maximum specified herein, an undesirably low Ms temperature can result and notch toughness is adversely affected. On the other hand, low amounts of nickel contribute to poor yield strength characteristics and also low notch toughness. Accordingly, it is advantageous that the nickel content not exceed 18.5% nor fall below about 16.5%.

High amounts of cobalt detract from notch toughness, whereas cobalt contents at the lower end of the cobalt range contribute to lower yield strength. It is thus beneficial to maintain the amount of cobalt within the range of about 11% to 14%.

Molybdenum, generally speaking, exerts a marked influence on lowering the Ms temperature, the result of which is incomplete transformation to martensite and low strength. However, provided that transformation would otherwise be complete, relatively high amounts of molybdenum (not above about 5%) can be used. It is advantageous that the molybdenum not exceed 4.25% and more advantageously it should not exceed about 4%. While the molybdenum content should not fall below 2.5%, for enhanced ductility and notch toughness at least 3% molybdenum should be present.

Titanium, and also aluminum, confers increased hardness and strength. As indicated above herein, usual metallurgical behavior might well dictate an unacceptable loss of ductility and toughness by any substantial increase in titanium above, say, the specific amounts used in US. Patent No. 3,093,519. Be that as it may, provided the chemistry of the steels is carefully controlled as required herein, no undue loss in these characteristics is experienced, notwithstanding that the steels are of comparatively high titanium content. It is to be underscored, however, that excessive amounts of titanium will detract from ductility and notch toughness while insuflicient titanium impairs yield strength. A titanium range of 1.4% to 2.1% is satisfactory and 1.6% to 2% is optimum. Aluminum should not exceed about 0.5% to avoid possible welding difficulties. A small amount of aluminum e.g., 0.05% or 0.1%, is beneficial, and for best results the maximum titanium plus aluminum should not exceed about 2.1%.

In obtaining an optimum combination of characteristics, including yield strength, tensile elongation, reduction in area and notch toughness, the following alloying ranges are most advantageous: about 17% to 18% nickel, about 12% to 13% cobalt, about 3.5% to 4% molybdenum, about 1.6% to 2% titanium, about 0.1% to 0.2% aluminum, up to 0.02% carbon, up to 0.1% manganese, up to 0.1% silicon and the balance essentially iron. An exemplary steel contains about 17.5% nickel, 12.5% cobalt, 3.7% molybdenum, 1.7% titanium, 0.15% aluminum, up to 0.02% carbon, up to 0.1% manganese, up to 0.1% silicon, balance essentially iron.

In carrying the invention into practice, utilization of vacuum processing is recommended together with alloylng ingredients of high purity. Before hot working, ingots should be soaked at a temperature of about 2200" F. to

about 2300 F. to achieve thorough homogenization. The hot working operation should generally be conducted over the range of about 1500 F. to 2000 F., e.g., 1700 F. to 1900 F., and it is preferred that the finishing temperature be as close to 1500 F. as is practicable.

Subsequent to cooling from hot working, the steels are preferably directly aged, although an annealing temperature can be employed prior thereto. A suitable aging temperature is from 800 F. to 1000 F. for a period up to about 24 hours, a shorter time period being used at the higher temperature. A temperature range of 850 F. to 950 F. is preferred, the time period being from about one half hour to ten hours. As is the case with most maraging steels, a most satisfactory maraging temperature is about 900 F. for about three hours. Temperatures above 1000 F. are unnecessary and not recommended in view of the possibility of occurrence of austenite reversion; however, an additional attribute of the steels is that they manifest substantial resistance to austenite reversion even at temperatures as high as 1050 F.

If an annealing operation must be employed, it should not be carried out at a temperature above 1700 F. and it is most preferred that the temperature be about 1400 F. Generally speaking, a loss of yield strength is experienced as a consequence of an annealing operation, the loss being greater the higher the annealing temperature.

For the purpose of giving those skilled in the art a better appreciation of the advantages of the invention, the following illustrative description and data are given.

A substantial number of steels were prepared, the compositions of which are set forth in Table I, Alloys Nos. 1 through 11 being within the invention and Alloys A, B and C being outside the scope thereof. The steels were produced by vacuum induction melting using aluminum deoxidation. Ingots were soaked one hour at 2300 F., forged and reheated to 2300 F., reforged to 2 x 2 inch or 1 x 3 inch bars, air cooled to room temperature, reheated to about 1800 F. to 1900 F. and hot rolled to inch bar.

TABLE I Percent Ni 00 Mo Ti Al 0 Fe 17. 5 11.8 3. 0 1. 54 0. 17 0.008 Ba]. 18.0 13.0 3. 6 1.88 0.08 0.007 13:11. 18.1 13. 0 3. 5 1. 62 0. 10 0.001 Ba]. 17. 5 11.5 4.15 1.48 0.62 0.01 Bal. 17.0 12. 3 4.07 1. 58 0.16 0.018 B211. 17. 4 7. 9 4. 97 1. 5 0.13 0. 008 B211. 18. 0 12. 0 3. 6 1. 9 0. 02 0. 004 13 al. 17. 2 12. 2 3. 1. 78 0. 23 0. 006 B al. 19. 7 15. 0 2. 05 1. 55 O. 15 0. 019 Bal. 17. 7 15. 0 3. 05 1.47 0. 13 0.013 Bal. 18.1 13.0 3.60 2. 25 0. 11 0.004 B al. 17. 6 19. 6 1. 4 1.57 0. 13 0. 009 13211. 19. 4 14. 9 1. 45 1. 55 0.13 0. 009 Bal. l4. 7 15. 3 1.50 1. l6 0. 76 0.012 Bal.

1 Alloys Nos. 5 and 7 contained 0.69% and 0.5% vanadium, respectively. Manganese and 51110011 contents of each alloy less than 0.05%.

Prior to test the steels were subjected to one or more of the following heat treatments:

Heat Treatment l.-Aged at 900 F. for 3 hours Heat Treatment II.Annealed at 1400 F. for one hour,

cooled and aged as in I Heat Treatment III.-Annealed at 1500 F. for one hour,

cooled and aged as in I The test results are reported in Table II, in which the yield strength (Y.S., 0.2% offset), ultimate tensile strength (U.T.S.) and notch tensile strength (N.T.S.) are given In thousands of pounds per square inch (K.S.I.) with tensile elongation (EL) and reduction in Area (R.A.) being given in percent Also included is the ratio of notch tensile strength to ultimate tensile strength (N.T.S./ U.T.S.), this being a well known indicator of notch toughness. In this connection, the notch bar (0.3 inch major diameter) specimens employed for this purpose had a stress concentration of 12.

plate has been cold rolled directly to more than 80% in reduction of thickness. In the form of sheet (0.070 inch TABLE 11 Heat Y.S U.T.S EL, R.A., N.T.S., N.T.S. treatment K. s.i K. s 1 percent percent K. s.i. U.'I.S

I 356 363 11 54 311 0. 86 II 344 352 6 32 288 0. 82 III 337 346 11 54 252 0. 73 I 359 367 7 32. 5 280 0. 76 II 349 358 8 42 178 0. 50 III 342 353 8 43 179 0. 51 I 343 352 11 57. 5 321 0. 91 II 339 348 8 45 298 0. 86 III 333 344 9 46. 5 265 0. 77 I 374 381 7 30. 5 176 O. 46 II 367 376 6 24. 5 231 0. 61 III 352 362 10 43. 5 191 0. 53 I 355 363 10 50 258 0. 71 II 336 345 9 45 190 0. 55 III 346 353 10 49 216 0. 61 III 342 349 8 40. 5 219 0. 63 I 355 364 8 44 261 0. 72 II 348 357 10 51. 5 254 0. 71 III 343 354 7 33 204 0. 58 I 349 356 8 37. 5 231 0. 65 II 348 356 10 46. 5 221 0. 62 III 336 351 8 41.0 170 0. 48 III 350 359 7 30. 5 157 0. 44 III 361 365 9 43. 5 120 0. 33 I 366 376 6 31. 5 192 0. 51 II 365 375 2. 6. 0 192 0. 51 III 358 368 3 7 144 0. 39 I III 72 1 III 326. 336 8. 5 37. 5 153 0.46 1 III 313. 1 325 8. 5 37. 5 86 0. 27

1 Alloys A, B and C aged 24 hours at 800 F. instead of 3 hours at 900 F.

2 Shattered The data tabulated in Tables I and II generally reflect that steels within the invention afford a satisfactory combination of yield strength and ductility. The fact that the ductility of Alloy No. 11 was comparatively lower is deemed attributable to the higher titanium content of 2.25%. As indicated hereinabove, it is advantageous that the titanium content not exceed about 2.1% and for optimum results it should not exceed 2%. In addition, the toughness of Alloys Nos. 1 through 8 was particularly good, each of these steels exhibiting a ratio of notch tensile strength to ultimate tensile strength above 0.5. The fact that the cobalt content of Alloys Nos. 9 and was on the high side contributed to lower toughness. As noted herein, for optimum toughness the cobalt should not exceed 14%. The high titanium content of Alloy No. 11 detracted from toughness as well as ductility. The data further show that yield strength decreases With annealing treatments of increasing temperature.

Alloys A, B and C are illustrative of the adverse effects encountered with low molybdenum, particularly with high cobalt (Alloy A) and nickel (Alloy B) contents. Further, Alloy C contained a comparatively low amount of titanium and this alloy exhibited about the lowest yield strength, notwithstanding that it contained 0.76% aluminum. This reflects that it is not only the sum of titanium and aluminum which is important but that with titanium contents below those recommended herein, inferior results can ensue irrespective of the fact that the total sum of titanium plus aluminum might otherwise be suflicient.

In addition to the fore-going, it is noteworthy of mention that alloys within the invention also possess attractive high temperature characteristics. Alloy No. 1, subjected to Heat Treatment II and tested at 1000 F., exhibited a yield strength of 194,000 p.s.i., an ultimate tensile strength of 229,000 p.s.i., a tensile elongation of and a reduction in area of 70% when tested at 1000 F. This alloy also showed remarkable resistance to austenite reversion, the alloy containing only 5.8% (X-ray determination) austenite after the application of an intentionally high aging treatment of three hours at 1050" F.

Moreover, the ultimate tensile strength of Alloy No. 1 when drawn to wire (0.025 inch diameter) and aged was 431,000 p.s.i. Further, wire has been drawn to 99.5% reduction in area without intermediate annealing and nominal thickness), Alloy No. 1 when given Heat Treatment III exhibited a yield strength of 350,000 p.s.i., and an autogenous, full penetration weld on the sheet material had a yield strength of 321,000 p.s.i. (after aging for three hours at 900 F.), a strength representing 92% of the base sheet.

In addition to being useful in the fabrication of pressure vessels, high strength cable and wire, high energy absorption springs, heavy duty tools and dies, the steels of the subject invention can also be used for fasteners and bearings.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A high strength maragin-g steel consisting essentially of from 15% to 20% nickel, from 11% to 16% cobalt, from 2.5% to 5% molybdenum, from 1.3% to 2.25% titanium, up to 0.8% aluminum, the sum of the titanium plus aluminum being from about 1.6% to 2.5 up to 0.05% carbon, up to 0.5% manganese, up to 0.5 silicon, up to 1% vanadium, up to 1% columbium, up to 5% chromium, up to 1% tantalum, up to 21% copper, up to 0.2% beryllium, up to 0.01% boron, up to 0.1% zirconium, the total amount of vanadium, columbium, chromium, tantalum, copper, beryllium, boron and zirconium not exceeding 7%, and the balance essentially 1I'OI1.

2. The steel set forth in claim 1 and containing about 16.5% to about 18.5% nickel, about 11% to 14% cobalt, about 3% to 4.25% molybdenum, about 1.4% to 2.1% titanium, up to 0.5% aluminum, with the sum of the titanium plus aluminum not exceeding 2.251%, and up to 0.03% carbon.

3. The steel as set forth in claim 1 and containing about 17% to 18% nickel, about 12% to 13% cobalt, about 3.5% to 4% molybdenum, about 1.6% to 2% titanium, about 0.1% to 0.2% aluminum, up to 0.02% carbon, up to 0.1% manganese and up to 0.1% silicon.

4. The steel as set forth in claim 1 and containing about 17.5% nickel, about 12.5% cobalt, about 3.7% molybdenum, about 1.7% titanium, about 0.15% aluminum,

up to about 0.01% carbon, up to about 0.02% manganese and up to about 0.02% silicon.

5. The steel as set forth in claim 1 in which the nickel, cobalt, molybdenum and titanium contents are correlated such that the following relationship is satisfied:

20 (percent Nil5) 11 (percent Co8) 68 (percent M02.5) 66.5 (percent Ti1.3) is not greater than about 247.

6. The steel as set forth in claim 2 in which the nickel, cobalt, molybdenum and titanium contents are correlated such that the following relationship is satisfied:

20 (percent Ni-15) 11 (percent Co8) 68 (percent Mo2.5) 66.5 (percent Ti1.3) is not greater than about 247.

7. A high strength maraging steel consisting essentially of from 15% to 20% nickel, from 8% to 16% cobalt, from 2.5% to 4.25% molybdenum, from 1.3% to 2.25% titanium, up to 0.8% aluminum, the sum of the titanium plus aluminum being from about 1.6% to 2.5%, up to 0.05% carbon, up to 0.5% manganese, up to 0.5% silicon, up to 1% vanadium, up to 1% columbium, up to 5% chromium, up to 1% tantalum, up to 2% copper, up to 0.2% berryllium, up to 0.01% boron, up to 0.1% zirconium, the total amount of vanadium, columbium, chromium, tantalum, copper, beryllium, boron and zirconium not exceeding 7%, and the balance essentially won.

8. The steel as set forth in claim 7 and containing about 11% to 14% cobalt, about 3% to 4% molybdenum and from 0.05% to 0.5% aluminum, the sum of the aluminum plus titanium not exceeding 2.25%.

References Cited UNITED STATES PATENTS 3,243,285 3/1966 Fragetta 75l23 3,166,406 1/1965 Flooreen 75124 3,132,938 5/1964 Decker 75124 HYLAND BIZOT, Primary Examiner.

U.S. Cl. X.R. 75-124, 128 

